The invention relates to apparatus for and a method of supporting a tubular member, such as a length of pipe, in a vertical orientation.
It is known to install an underwater pipeline from a drill rig by what is known as a xe2x80x9cJ-layxe2x80x9d method. In this method of installation lengths of pipe are supported vertically while the individual lengths are welded together. The welded pipe is subsequently lowered towards the seabed through the rotary table on the drill rig. Thus, the pipe is lowered vertically from the drill rig and curves towards a generally horizontal orientation on the seabed.
The most convenient method of installing pipeline by a drilling rig is to attach a flange (or J-lay) collar to the pipe, and use the J-lay installation method. However, there are problems in using this method to install Steel Catenary Risers (SCR) which are attached to a floating production system such as a semi-submersible or a Floating Production Storage and Offloading vessel (FPSO). This problem is caused by the motion of the vessel and environmental loading during the operational lifetime of the SCR. Both of these may cause undesirable stressing at the stress concentrated locations adjacent to the collar flange attachments. This effect becomes more severe in harsher environmental conditions and stress levels become unacceptable due to the fatigue life limitation.
Installation of a SCR using the J-lay collar was performed in the Gulf of Mexico with a Tensioned Leg Platform (TLP). In a TLP, although the platform is semi-submersible, the platform is secured to the sea bed by tethers which are in tension between the sea bed and the platform. Hence, the tension in the tethers helps to minimise the lateral movement of the TLP and substantially eliminates the vertical movement. In addition, the sea state in the Gulf of Mexico is generally calm which also helps to minimise the movement of the TLP.
However, this arrangement for installation of SCRs is not suitable for a floating production system which incorporates anchor lines. In this type of platform, the movement of the platform can result in high flexing of the pipes which produces high cyclic stress loading in the area of the flanges which can potentially result in failure of the pipe. In addition, the arrangement is also unlikely to be suitable for a TLP in sea conditions more adverse than the Gulf of Mexico, such as the North Sea, where the movement of a TLP would be greater and could result in fatigue and failure of the pipe due to the high stress zones around the flanges attached to the pipe.
In accordance with a first aspect of the present invention, there is provided apparatus for supporting a tubular member comprising a first member comprising a first formation, the first member being adapted to be attached to the tubular member, and a detachable member adapted to engage the first formation; the detachable member having an internal formation which engages with the first formation, and an external formation for engagement by a lifting device to support the tubular member when the detachable member is placed against the tubular member and the internal formation is engaged with the first formation.
Preferably, the detachable member is also adapted to engage the tubular member adjacent the first formation.
In accordance with a second aspect of the present invention, there is provided a method of supporting a tubular member comprising attaching to the tubular member a first member comprising a first formation, placing a detachable member against the first formation so that the first formation engages an internal formation on the detachable member; and engaging an external formation on the detachable member with a lifting device to support the tubular member.
Optionally, the detachable member is also placed around a portion of the tubular member adjacent the first formation.
The first member typically comprises a collar and is typically coupled to the tubular member by any conventional means, such as by welding. The first formation can be in the form of a protrusion on the collar.
The detachable member is typically engageable with the collar.
An advantage of the invention is that by using a detachable member to engage with a first formation attached to the tubular member, it is possible to reduce the size of the first formation and therefore, decrease the stress concentration in the pipe adjacent to the first formation.
Preferably, the method further comprises the step of fixing the tubular member or the first member to another tubular member and subsequently detaching the detachable member from the first tubular member.
Preferably, the tubular member is supported with the central longitudinal axis of the tubular member in a substantially vertical orientation.
In accordance with a third aspect of the present invention, there is provided a method of laying underwater pipeline comprising a method of supporting a first tubular member in accordance with the second aspect of the present invention, attaching another tubular member also supported by a method according to the second aspect of the present invention to the first tubular member or the first member, such that the tubular members are in end to end relationship and the central longitudinal axis of the tubular members are substantially coincident, releasing the detachable member against the first formation on the first tubular member, and lowering the tubular members by means of the lifting device attached to the second detachable member to lower the tubular members towards the seabed.
Preferably, the first member may be attached to an end of the tubular member. However, in an alternative example of the invention, it is possible that the first member may be attached to an external surface of the first tubular member. Typically, the first member is attached to the tubular member by a welding operation, and a welding operation may also be used to attach the other tubular member to the first tubular member or the first member.
Preferably, the method of laying an underwater pipeline in accordance with a third aspect of the invention comprises laying the underwater pipeline from a semi-submersible vessel or floating vessel, such as a semi-submersible drilling rig, a floating production platform or a tensioned leg platform.
In one example of the invention, the first formation may be removed from the first member, for example by a reaming or machining operation, after detachment of the detachable member. This may be useful for applications where strictly no stress concentration condition along the SCR is permitted.
In one example of the invention the detachable member may be adapted to rest on a support pedestal located on the drill floor of the platform or vessel. Alternatively, the detachable member may be adapted to engage with and be supported by slips located in a rotary table on the platform or vessel.
Typically, the detachable member is hinged along an axis substantially parallel to the central longitudinal axis of the detachable member and the tubular member. Preferably, when placed around the formation and the portion of the tubular member adjacent to the formation, the detachable member is clamped to the first formation and the portion of the tubular member adjacent the first formation.